一次梅雨锋特大暴雨过程的数值模拟研究:不同尺度天气系统的影响作用

NUMERICAL SIMULATION OF A HEAVY RAINFALL EVENT ALONG THE MEIYU FRONT: INFLUENCES OF DIFFERENT SCALE WEATHER SYSTEMS

2003年7月4～5日在江淮地区沿梅雨锋有一系列中尺度对流系统相继生成和强烈发展,导致了江淮地区特大暴雨的形成.该研究利用中尺度数值模式MM5对这次梅雨锋暴雨过程进行了数值模拟,在模拟结果的基础上重点分析了不同尺度天气系统相互作用对这次特大暴雨过程的影响作用.在这次特大暴雨过程中,位于梅雨锋北侧的东北-西南走向深厚、稳定的短波槽系统与槽前从西南移来的低涡系统相配合,加强了位于梅雨锋北侧的反气旋性扰动发展,从而导致梅雨锋北侧反气旋性涡旋的形成.该类反气旋性涡旋形成对江淮切变线的加强与维持起重要作用.中尺度对流系统的潜热释放首先导致梅雨锋低层切变线上的中尺度对流性涡旋(MCV)的形成,而中尺度对流性涡旋的形成进一步加强了切变线上的低层辐合,中尺度对流性涡旋消亡后,在切变线上形成低涡.梅雨锋附近主要存在4种不同垂直环流,它在降水的不同阶段具有不同的结构、配置与动力学作用.其中跨锋面、高层非地转两支垂直环流对锋区的对流扰动发展和暴雨形成最为重要,而降水发展可以调整锋区垂直环流的结构、配置,随降水的减弱,梅雨锋区的不同垂直环流系统又重新恢复到先前结构.梅雨锋上不同尺度、高度的天气系统之间的相互作用主要通过这些垂直环流系统调整实现.

A heavy rainfall event, mainly caused by the successive mesoscale convective systems （MCSs） along the meiyu front, occurred within the mid-lower reaches of the Yangtze river on 4-5 July, 2003. A mesoscale numerical model of the Penn State/NCAR Mesoscale Model Version 5 （MMS） was employed to numerical simulate this heavy rainfall event. The stable, deep northeast-southwest oriented short-wave trough to the north of the meiyu front coupled with the low vortex systems moved from the southwest was favorable to the genesis and development of the anticyclonic disturbance located northward to the meiyu front. And the subsequent formation and enhancement of a lower anti-cyclonic vortex, was expected to be crucial for the initiation and maintenance of the low-level shear line and the low-level vortex along the meiyu front. The latent heating from the MCSs along the Meiyu front resulted in the formation of mesoscale convective vortex （MCV）, and meanwhile the MCV enhanced the low-level convergence near the low-level shear line and thus lead to the initiation of the low-level vortex embedded in the low-level shear line. There were four different vertical circulations within the meiyu front, which were important for the interactions among the different scales weather systems in both the upper- and lower-levels. These vertical circulations possessed the different structures and dynamic roles at different rainfall stages. The development of convection disturbance within the meiyu frontal system and the initiation of subsequent heavy rainfall were associated with the vertical circulation across the front and upper level ageostrophic vertical circulation, while the development of the convection would modify the vertical circulation structure. At last when the heavy rainfall weakened the ver- circulation would restore again.